If the two clients have completely different goals, vastly different capabilities (through joint issues,health issues, etc) and abilities (through lack of experience) then it’s more advisable to train separately through private one-to-one sessions until issues have been addressed and abilities have increased.
A general guide for these sessions are that you need to have similar goals and have similar capabilities and abilities.
Some methods used include free weights, machine weights, body weight, ViPr, Tabata, plyometrics, HIIT, cardio.